That worrisome finding comes from a study of 49 North Dakotans who suffered at least some symptoms after a West Nile infection in 2003.

Four of five people who get West Nile virusWest Nile virus don't have any symptoms.

One in five, however, gets a fluflu-like illness called West Nile fever. It's supposed to be what doctors call a "benign, self-limited illness."

But it may be much worse, University of North Dakota researcher Paul J. Carson, MD, and colleagues find.

"Our patients continued to show clinical abnormalities one year after illness," the researchers report. "Patients with 'milder' illness -- that is, non-hospitalized patients with West Nile fever -- demonstrated equivalent rates of these abnormalities."

This means West Nile fever may leave lasting damage in its wake, Carson and colleagues suggest. They report their findings in the Sept. 15 issue of Clinical Infectious Diseases.

Lingering West Nile Symptoms

The year 2003 was a big one for West Nile virus in the U.S. Nearly 10,000 confirmed infections were reported to the CDC -- 617 of them in North Dakota.

Carson's team tried to track down the 122 North Dakota patients known to the state at the time of their study. All of these people reported some kind of West Nile illness -- either West Nile fever or more severe West Nile encephalitisencephalitis.

The researchers were able to persuade 49 of these patients to submit to a battery of tests, including measures of psychological and neurological function. What they found was scary.

One patient had the most severe kind of West Nile illness: polio-like paralysis.

About a fifth of the study's patients had a confirmed West Nile infection of the brain or lining of the brain and spinal cord (encephalitis or meningitismeningitis).

The other four-fifths had West Nile fever.

About a third of the patients had been hospitalized.

A year after their infection, the West Nile patients commonly reported fatigue, memory problems, weakness, headache, joint pain, and balance problems.

Half reported poor physical health. Nearly one in four reported depression. One in five developed a tremor.